Process and plant for recovering acrylic acid

ABSTRACT

The present invention relates to the process for recovering acrylic acid, comprising the steps
         a) division of a heated mother acid stream in direction of an absorption column ( 201 ) and a dissociation column ( 205 ),   b) feeding of a heated mother acid substream as runback to the dissociation column ( 205 ),   c) feeding-in of at least one stripping gas stream to the dissociation column ( 205 ),   d) feeding-in of a secondary component stream comprising oligomeric acrylic acid from the condensation column ( 201 ) to the dissociation column ( 205 ),   e) dissociation of part of oligomeric acrylic acid in the dissociation column ( 205 ) to give monomeric acrylic acid,   f) removal of secondary components comprised in the secondary component stream in the dissociation column ( 205 ),   g) discharge of monomeric acrylic acid as gas mixture with introduced circulating stripping gas stream from the dissociation column ( 205 ) and   h) feeding-in of the gas mixture to the condensation column ( 201 ).

This patent application claims the benefit of pending U.S. provisionalpatent application Ser. No. 62/057,267 and DE patent application SerialNumber DE 10 2014 114 193.8, both filed on Sep. 30, 2014, incorporatedin their entirety herein by reference.

The present invention relates to a process and a plant for recoveringacrylic acid.

Acrylic acid is an important basic chemical. Owing to its very reactivedouble bond and also the acid function, it is, in particular as monomer,suitable for preparing polymers.

Of the amount of acrylic acid monomer produced, the major part is, forexample, esterified before polymerization (e.g. to give adhesives,dispersions or surface coating compositions). Only a minor part ispolymerized directly (e.g. to give “superabsorbents”). While monomer ofhigh purity is generally required in the direct polymerization ofacrylic acid, the requirements in respect of the purity of the acrylicacid are not as demanding when it is esterified before polymerization.

Acrylic acid can be obtained, inter alia, by heterogeneously catalyzedgas-phase partial oxidation of C₃ precursors of acrylic acid by means ofmolecular oxygen over solid-state catalysts at elevated temperature. Theterm C₃ precursors encompasses chemical compounds which are formallyobtainable by reduction of acrylic acid. In the preparative process,these C₃ precursors are passed in the gaseous state, generally dilutedwith inert gases such as nitrogen, CO₂, saturated hydrocarbons and/orsteam, in a mixture with molecular oxygen at elevated temperatures andoptionally superatmospheric pressure over transition metal mixed oxidecatalysts and oxidatively converted into a product gas mixture whichcomprises acrylic acid and secondary components such as furfurals,benzaldehyde and maleic anhydride and from which the acrylic acid has tobe separated off.

The acrylic acid obtained is not a pure product but instead a mixturecomprising not only acrylic acid (in general ≧90%, or ≧95% of the totalweight) but also typical by-products of the gas-phase oxidation, e.g.water, lower aldehydes (e.g. furfurals, acroleins or methacrolein,benzaldehyde), lower carboxylic acids (e.g. acetic acid, propionoicacid), etc., and oligomers of acrylic acid.

The cause of the formation of oligomers of acrylic acid is that acrylicacid present in the condensed phase forms acrylic acid oligomers(Michael adducts) by reversible Michael addition onto itself and ontothe dimer which is formed, and also oligomers formed by free-radicalpolymerization. The presence of water, the unavoidable by-product of agas-phase catalytic oxidative preparation of acrylic acid, and elevatedtemperatures promote the formation of oligomers of acrylic acid.

Since the respective oligomers have a higher boiling point than acrylicacid, they accumulate in the high boiler fraction (e.g. in the liquidbottoms) both in an isolation of acrylic acid by distillation and in afractional condensation of the product gas mixture from a gas-phasecatalytic oxidative preparation.

DE 199 24 533 A1 discloses a process as described above for preparingacrylic acid, in which a basic isolation of a crude acrylic acid iscarried out by fractional condensation of the product gas mixture fromthe heterogeneously catalyzed gas-phase partial oxidation. Aredissociation of the acrylic acid oligomers comprised in the output ofa quenching liquid is said to be integrated in such a way that the timeon stream of the process, in particular of the fractional condensation,is not decreased significantly. The redissociation of the acrylic acidoligomers has the aim of increasing the yield of product of value. DE199 24 533 A1 provides a circulation reactor for the dissociation.

DE 102 47 240 A1 describes a very similar process for preparing acrylicacid, in which a product gas mixture comprising acrylic acid is firstlycooled by direct cooling with a quenching liquid and the cooled productgas mixture is subsequently fed into a condensation column equipped withseparation-active internals. The crude acrylic acid is taken off fromthe condensation column and passed to a further purification bycrystallization. The mother liquor obtained in this purification bycrystallization is recirculated in its entirety to the condensationcolumn. Liquid bottoms comprising acrylic acid oligomers are taken fromthe bottom of the condensation column and used as quenching liquid. Thepart of the quenching liquid which is not vaporized during cooling ofthe product gas mixture is circulated via the bottom and optionally viaa heat exchanger and part of the quenching liquid is discharged asoutput from this circuit and fed to a dissociation vessel forredissociation. The acrylic acid-comprising dissociation gases which aregiven off in gaseous form are recirculated to the circuit of thequenching liquid or to the condensation column or to the circuit of thequenching liquid and to the condensation column, with the dissociationgases being subjected to a countercurrent rectification and at leastpartial condensation before being recirculated; here, the amount ofcondensate formed corresponds to at least the amount of runbacknecessary for the countercurrent rectification.

The processes according to the prior art are in themselves advantageousand lead to appropriate yields of product of value, i.e. of acrylicacid. However, these increased yields are made possible only by means ofa high outlay in terms of apparatus and energy consumption.

In view of this background, it is an object of the present invention toindicate a process for recovering acrylic acid and provide acorresponding plant, the two of which increase the yield of product ofvalue further compared to the prior but at the same time allow theprocess to be carried out more efficiently in respect of the outlay interms of apparatus and energy.

The above object is achieved, in a first aspect of the presentinvention, by a process for recovering acrylic acid, which comprises thesteps of

-   -   a) division of a heated mother acid stream from a        crystallization apparatus in the direction of an absorption        column (201) and in the direction of a dissociation column        (205),    -   b) feeding of the first heated mother acid substream as runback        to the uppermost tray of the dissociation column (205),    -   c) feeding-in of at least one stripping gas stream below the        lowermost tray of the dissociation column (205),    -   d) feeding-in of a secondary component stream comprising        oligomeric acrylic acid from the condensation column (201) to a        middle tray of the dissociation column (205),    -   e) dissociation of at least part of the oligomeric acrylic acid        from the secondary component stream in the dissociation column        (205) to give monomeric acrylic acid,    -   f) removal of the secondary components comprised in the        secondary component stream by countercurrent rectification in        the superposed dissociation column (205),    -   g) discharge of the monomeric acrylic acid without condensation        as gas mixture with the introduced circulating stripping gas        stream at the top of the dissociation column (205) and    -   h) feeding-in of the gas mixture below the lowermost tray of the        condensation column (201).

In a second aspect of the present invention, the above object isachieved by a plant (1) for recovering acrylic acid, which comprises

-   -   a condensation column (201),    -   a dissociation column (205),    -   a first line (101) connected to the dissociation column (205),    -   a second line (102) connecting the condensation column (201) and        the dissociation column (205),    -   a third line (103) feeding a substream of the mother acid        obtained in the crystallization into the dissociation column        (205),    -   a fourth line (104) connecting the crystallization apparatus and        the absorption column (201) and    -   a fifth line (105) connecting the dissociation column (205) and        the condensation column (201).

The present invention has the substantial advantage that the process ofthe invention and the plant (1) of the invention allow secondarycomponents formed in the preparation of acrylic acid, in particularoligomeric acrylic acid, to be dissociated and recirculated withimproved efficiently to the preparative process, as a result of whichthe yield of product of value, i.e. of acrylic acid, and its purity canbe increased.

The present invention is described in detail below.

Where reference is made to process features in the context of the plant(1) of the invention in the description below, these preferably relateto the process of the invention. Likewise, substantive features whichcan be carried out in the context of the process of the inventionpreferably relate to the plant (1) of the invention.

In a first aspect of the present invention, the abovementioned object isachieved by a process for recovering acrylic acid. The process of theinvention comprises the steps a) to h) and is described below.

In a step a), a heated mother acid stream from a crystallizationapparatus is divided into two substreams in the direction of anabsorption column (201) and in the direction of a dissociation column(205). For the purposes of the present invention, “heated” means thatthe mother acid stream or the mother acid substreams has/have, afterdischarge from the crystallization apparatus, been heated to atemperature of preferably from 50° C. to 100° C., more preferably from60° C. to 90° C., in particular from 70° C. to 80° C. The configurationaccording to the invention of the process is not limited to a particularcrystallization process by means of which acrylic acid is purified bypartial crystallization, separation of the frozen-out acrylic acid fromthe liquid (mother acid) comprising the impurities and melting of thepure acrylic acid crystals which have been separated off. Here, forexample, it is possible to use a falling film crystallization or asuspension crystallization as combination of cooling plate crystallizersand scrubbing columns, with the latter process variant being preferred.

In the context of the present invention, the term “mother acid” (incomparable documents sometimes also referred to as “mother liquor”)refers to a solution of acrylic acid which, after the pure product hasbeen separated off in a crystallization apparatus, comprises theimpurities separated off in the crystallization apparatus, with theproportion by weight of acrylic acid in the mother acid being ≧80% byweight.

In step b), a first heated mother acid substream is fed to the uppermosttray of the dissociation column (205) having 45 trays. The dissociationcolumn (205) is, according to the present invention, preferably equippedwith dual-flow trays as separation-effective internals. A second heatedmother acid substream is introduced onto tray 18 of the absorptioncolumn (201) comprising 75 trays.

A step c) provides for introduction of at least one stripping gas streambelow the uppermost tray of the dissociation column (205). Here, thestripping gas stream is preferably directed onto the liquid surface ofthe liquid phase. The stripping gas stream is, in particular, used ascirculating gas. For the purposes of the present invention, “circulatinggas” is a gas which serves for dilution of the starting materials anduptake of heat of reaction in the gas-phase oxidation and is essentiallyinert in the gas-phase reaction. The circulating gas comprisesessentially nitrogen and, at a concentration of <5% by volume, oxygen,water vapor, carbon oxides and mixtures thereof and very small amounts(<0.8% by volume) of ethylene, ethane, propene, propane, acrolein,acrylic acid and acetic acid.

In a step d), a secondary component stream comprising oligomeric acrylicacid from the condensation column (201) is fed to a middle tray of thedissociation column (205). This middle tray is, in particular, a tray inthe tray region 8 and 10.

In a step e), at least part of the oligomeric acrylic acid in thesecondary component stream is dissociated in the dissociation column(205) to give monomeric acrylic acid. This preferably occurs thermallyat temperatures of >150° C. The dissociation can be accelerated byaddition of small amounts of sodium hydroxide or amines to the liquidphase in the dissociation column.

Step f) provides for removal of the secondary components comprised inthe secondary component stream by countercurrent rectification in thesuperposed dissociation column (205).

The monomeric acrylic acid obtained in step e) is, in a step g),discharged without condensation as gas mixture together with theintroduced circulating stripping gas stream at the top of thedissociation column (205) and the gas mixture is subsequently, in a steph), fed in below the uppermost tray of the condensation column (201).

The monomeric acrylic acid is thus recirculated to the process in anadvantageous way. Since the acrylic acid is introduced in gaseous formand does not have to be vaporized first there, more energy is availablefor separation of the acrylic acid from secondary components in thecondensation column (201). According to the present invention, thecondensation column (201) is preferably configured as a tray column. Itis preferably equipped with dual-flow trays in the lower region, withThormann trays in the middle region and with valve trays in the upperregion.

The process of the invention has the advantage over the prior art thatpartial condensation of the acrylic acid stripped out in thedissociation column (205) to provide runback for the rectificationcolumn is dispensed with and part of the molar acid stream produced inthe crystallization apparatus is used instead of the runback produced bycondensation of the vapor at the top of the dissociation column (205).As a result, a condensation unit at the top of the dissociation column(205) can be omitted, which reduces the outlay in terms of apparatus. Inaddition, it is not necessary to provide cooling power for condensation.Furthermore, the energy introduced via a bottom heat exchanger of thedissociation column (205) is additionally available in the condensationcolumn (201), which improves the separation of acrylic acid fromsecondary components.

A further advantage of the present process is that monomeric acrylicacid can be recovered in an improved yield from the respective oligomersby means of the process of the invention and can be returned as lowboiler to the overall process. The term “oligomeric acrylic acid”refers, in particular, to dimers and trimers of acrylic acid. Comparedto the prior art, the work-up loss of acrylic acid is reduced by 0.3%.For the purposes of the present patent application, the term “work-uploss” refers to the proportion of acrylic acid which, based on theacrylic acid supplied from the synthesis, is not separated off from thesecondary components and cannot be obtained as product.

The process of the invention can advantageously be carried out by meansof a plant (1) according to the invention, which is described below.

In an embodiment of the process of the invention, it has been found tobe advantageous for the mother acid stream to be heated, in particular,against an acrylic acid stream from a condensation column (201). Theacrylic acid stream is preferably taken off as target product via a sideofftake of the condensation column (201) and essentially fed to acrystallization apparatus. This acrylic acid stream is hot (i.e. from95° C. to 100° C.) and highly concentrated (i.e. from 95% by weight to98% by weight of acrylic acid) and has to be cooled before entering thecrystallization apparatus.

According to the invention, the heat energy present in the acrylic acidstream is therefore transferred by means of a heat exchanger to themother acid stream and thus introduced into the two mother acidsubstreams. This energy is thus available both in the absorption column(201) for fractionation of the acrylic acid and in the dissociationcolumn (205) for dissociation of the dimeric acrylic acid, or lessenergy has to be introduced into the dissociation via a bottom heatexchanger present on the dissociation column (205).

In a preferred embodiment, step h) is carried out indirectly by the gasmixture being introduced into a quenching apparatus (203) for quenchinga product gas mixture comprising acrylic acid.

The mother acid stream taken off from the crystallization apparatuspreferably comprises essentially acrylic acid and proportions of waterand acetic acid, in particular from 90% by weight to 95% by weight ofacrylic acid, from 3% by weight to 6% by weight of water and from 1% byweight to 2% by weight of acetic acid and also small proportions (ineach case <0.5% by weight) of formaldehyde, propionoic acid, furfural,maleic acid and diacrylic acid. The mother acid stream is taken off witha temperature just above the crystallization temperature of acrylic acid(from 15° C. to 20° C.) and preferably preheated to the correspondingthermodynamic equilibrium temperature at the position of the point ofintroduction into the absorption column (201) of about 80° C. in orderto ensure very effective separation.

Furthermore, the secondary component stream preferably comprisesessentially acrylic acid, diacrylic acid and polyacrylic acid and alsoproportions of maleic acid, benzoic acid, benzaldehyde, furfurals andwater, in particular from 50% by weight to 60% by weight of acrylic acidor methacrylic acid, from 20% by weight to 30% by weight of diacrylicacid and from 5% by weight to 10% by weight of polyacrylic acid and alsofrom 6% by weight to 9% by weight of maleic acid, from 1% by weight to2% by weight of benzoic acid, from 0.5% by weight to 1% by weight ofwater and from 0.5% by weight to 1% by weight of 4-methoxyphenol, alsosmall proportions (in each case <0.5% by weight) of acetic acid,furfural, benzaldehyde, phthalic anhydride, phenothiazine and diacrylicacid. The secondary component stream preferably has a temperature offrom 100° C. to 130° C., in particular from 105° C. to 115° C., in orderfirstly to achieve sufficient preconcentration of the bottom liquid fromthe absorption column (201) before transfer into the dissociation column(205) and secondly to limit dimer formation in the bottom region of theabsorption column (201).

In addition, the stripping gas stream can comprise essentially nitrogen,acrylic acid, water and oxygen and also proportions of carbon dioxideand acetic acid, in particular from 80% by weight to 85% by weight ofnitrogen, from 3% by weight to 5% by weight of acrylic acid, from 3% byweight to 5% by weight of water, from 3% by weight to 4% by weight ofoxygen, from 2% by weight to 3% by weight of carbon dioxide and from 1%by weight to 2% by weight of acetic acid and also small proportions (ineach case <0.7% by weight) of carbon monoxide, acrolein or methacrolein,formic acid, propene and propane. The stripping gas stream preferablyhas a temperature of from 80° C. to 90° C., in particular about 85° C.

It has been found to be advantageous in terms of the efficiency of theprocess of the invention, in particular for the yield of product ofvalue, for from 60% to 95%, in particular from 85% to 90%, of thedissociable components of the secondary component stream to bedissociated in step e). Preference is given to essentially the dimersand trimers of acrylic acid being dissociated. As regards the yields ofproduct of value, higher degrees of dissociation of 95% areadvantageous, but are technically difficult to control since atdissociation yields of >95% the remaining product tends to undergomassive solids formation and is thus very difficult to handle.

In a second aspect of the present invention, the abovementioned objectis achieved by a plant (1) for recovering acrylic acid. The plant (1)according to the invention comprises a condensation column (201) and adissociation column (205).

What is meant by the condensation column (201) and the dissociationcolumn (205) for the purposes of the present invention has beendescribed above in relation to the process of the invention.

The plant (1) further comprises a first line (101) which is connected tothe dissociation column (205) and feeds at least one gas stream ascirculating stripping gas to the bottom region of the dissociationcolumn (205). A second line (102) connects the condensation column (201)and the dissociation column (205) and conveys a secondary componentstream from the condensation column (201) to the dissociation column(205).

A third line (103) serves for transfer of a substream of the mother acidobtained in the crystallization to the dissociation column (205). Acrystallization apparatus and the absorption column (201) are connectedby a fourth line (104) and the dissociation column (205) and thecondensation column (201) are connected by a fifth line (105).

Corresponding definitions of the elements of the plant (1) and termsused have been given above in relation to the process of the inventionand also apply to this plant (1).

The advantages of the plant (1) of the invention are essentially thesame as for the above-described process of the invention. The presentplant (1) creates the prerequisites in terms of apparatus for dividingthe mother acid stream to give two mother acid substreams and feed theseto the condensation column (201) and the dissociation column (205). Inthis way, no energy is taken off from the overall process but insteadremains in the process. The provision of additional heat exchangers forintroduction of external energy is greatly reduced.

In addition, plant parts and apparatuses provided in plants of the typein question in the prior art can be saved, for example surfacecondensers or spray condensers with associated pumps and heatexchangers, also devices for metering in inhibitors. In ongoingoperation of the plant, the amounts of inhibitors for avoidingpolymerization of acrylic acid can be reduced since the use of motheracid as runback to the dissociation column (205) already provides asufficiently stabilized acrylic acid.

In an embodiment of the plant (1) of the invention, this plant furthercomprises a quenching apparatus (203) for quenching a product gasmixture comprising acrylic acid, which is arranged in the fifth line(105) between the dissociation column (205) and the condensation column(201). This provides a physical means of efficiently exploiting the gasmixture from the dissociation column (205) and its temperature forquenching the hot product gas mixture.

The plant (1) of the invention is particularly advantageous when it isintegrated into an overall plant for the preparation of acrylic acid. Asindicated above, the plant (1) of the invention can increase the totalefficiency of acrylic acid production while at the same time reducingthe outlay in terms of apparatus.

Further objectives, features, advantages and possible uses can bederived from the following description of examples of the invention withthe aid of the figure. Here, all features described and/or depictedform, in themselves or in any combination, the subject matter of theinvention, even independently of their summary in the claims or theirback-reference. The drawing shows:

FIG. 1 a schematic depiction of the plant 1 according to the inventionin one embodiment of the invention.

FIG. 1 schematically shows the plant 1 according to the invention in oneembodiment of the invention. Central elements here are the condensationcolumn 201 and the dissociation column 205.

From a part of the plant which is not shown and in which theheterogeneously catalyzed gas-phase partial oxidation is carried out, ahot product gas stream is fed in with a temperature of about 270° C.This is introduced into a quenching apparatus 203 in order to suppresspossible reactions of its constituents and to reduce its temperature. Agas mixture from the dissociation column 205 having a temperature ofabout 95° C. is likewise fed via a fifth line 105 into the quenchingapparatus 203. This gas mixture will be described in more detail later.

The gases and liquids fed into the quenching apparatus 203 are, afterleaving the latter, fed into the bottom region of the absorption column201. In this absorption column 201, the acrylic acid produced isseparated off from the product mixture by absorption and taken off fromthe absorption column 201 via a side offtake. The acrylic acid streamtaken off is highly concentrated (about 97% of acrylic acid) and has atemperature of about 99° C. This acrylic acid stream (also referred toas “crude acrylic acid”) is fed via a series of apparatuses which willnot be described in more detail here to a crystallization apparatus inwhich purification of the acrylic acid by crystallization is carriedout. Apart from highly pure crystalline acrylic acid, mother acid alsoremains in the crystallization apparatus and this is taken off as motheracid stream via a line 104.

While according to the prior art (see, for example, DE 102 47 240 A1)this mother acid stream is fed directly to a lower tray of an absorptioncolumn (comparable to absorption column 201), it is a feature of theinvention that the mother acid stream is thermally coupled against theacrylic acid stream taken off the absorption column 201. The mother acidstream initially has a temperature of about 20° C. and is heated toabout 93° C. by the thermal coupling. In this way, the excess heatenergy of the acrylic acid stream can be transferred to the mother acidstream.

The heated mother acid stream is divided into a first mother acidsubstream and a second mother acid substream. While the second motheracid substream is fed via line 108 to the absorption column 201, thefirst mother acid substream is fed as runback via a third line 103 tothe uppermost tray of the dissociation column 205 and the heat energy ofthe acrylic acid stream is thus indirectly introduced into thedissociation reaction.

The dissociation column 205 is supplied via a first line 101 with astripping gas stream as circulating gas from a plant section, which willnot be described in more detail, below the lowermost tray. Thisstripping gas stream has a temperature of about 85° C. At a middle trayof the dissociation column 205, a secondary component stream comprisingoligomeric acrylic acid and having a temperature of about 109° C. is fedin via a second line 102 from the bottom of the absorption column 201.

The secondary component stream comprises high boilers such asbenzaldehyde, furfural and maleic acid. However, the major constituentof the secondary component stream is acrylic acid together witholigomers thereof and polyacrylic acid. These secondary components, inparticular the acrylic acid oligomers, are redissociated in thedissociation column 205 and taken off as low boiler fraction togetherwith the circulating gas via the top of the column. This low boilerfraction forms the gas mixture and is fed via line 105 to the quenchingapparatus 203. The high-boiling components, in particular, remain in thebottom of the dissociation column 205 and are taken off and passed todisposal.

In the present example, the yield of acrylic acid as product of value issignificantly increased by redissociation of the acrylic acid oligomersand recirculation to the condensation column 201. The process of theinvention and the plant 1 according to the invention make it possible toproduce an amount of 20.4 t/h of acrylic acid.

A specific embodiment is described below for the example of thepreparation of acrylic acid.

Example (the steady state is described)

A heterogeneously catalyzed gas-phase oxidation of propylene of “polymergrade” purity gave a product gas mixture having a temperature of 301° C.and the following composition:

12.168% by weight of acrylic acid,

0.242% by weight of acetic acid,

5.281% by weight of water,

0.035% by weight of formic acid,

0.18% by weight of formaldehyde,

0.068% by weight of acrolein,

0.005% by weight of propionoic acid,

0.003% by weight of furfurals,

0.001% by weight of allyl acrylate,

0.0005% by weight of allyl formate,

0.013% by weight of benzaldehyde,

0.148% by weight of maleic anhydride,

0.011% by weight of benzoic acid,

0.011% by weight of phthalic anhydride,

2.126% by weight of CO₂ ,

0.658% by weight of CO,

0.08% by weight of propane,

0.174% by weight of propylene,

3.06% by weight of oxygen and

75.728% by weight of nitrogen.

Further constituents are not detected.

The product gas mixture (176 610 kg/h) is cooled to a temperature of120.1° C. by direct cooling in a spray cooler operated cocurrently. Theliquid used for the direct cooling is a mixture of bottom liquid fromthe absorption column 201 and high boiler fraction taken off from thefirst collection tray closing off the bottom region of this absorptioncolumn 201. The composition of the bottom liquid is:

35.59% by weight of acrylic acid,

0.16% by weight of acetic acid,

0.71% by weight of water,

0.01% by weight of formic acid,

<0.001% by weight of formaldehyde,

0.01% by weight of acrolein,

0.04% by weight of propionoic acid,

0.21% by weight of furfurals,

0.001% by weight of allyl acrylate,

<0.001% by weight of allyl formate,

0.68% by weight of benzaldehyde,

10.56% by weight of maleic anhydride,

0.683% by weight of benzoic acid,

0.77% by weight of phthalic anhydride,

41.09% by weight of diacrylic acid,

8.0% by weight of polyacrylic acid (Michael adducts),

0.34% by weight of phenothiazine,

0.82% by weight of MEHQ,

0.59% by weight of other high-boiling constituents and

<0.001% by weight of oxygen.

The high boiler fraction has the following composition:

86.62% by weight of acrylic acid,

0.29% by weight of acetic acid,

1.32% by weight of water,

0.02% by weight of formic acid,

0.002% by weight of formaldehyde,

0.011% by weight of acrolein,

0.09% by weight of propionoic acid,

0.42% by weight of furfurals,

0.002% by weight of allyl acrylate,

0.001% by weight of allyl formate,

1.03% by weight of benzaldehyde,

8.39% by weight of maleic anhydride,

0.03% by weight of benzoic acid,

0.02% by weight of phthalic anhydride,

1.61% by weight of diacrylic acid,

0.017% by weight of phenothiazine,

0.07% by weight of MEHQ and

0.0004% by weight of oxygen.

The amount of the high boiler fraction taken off is 79 528 kg/h. It istaken off at a temperature of 105° C. and fed at this temperature to thespray cooler. The amount of the bottom liquid taken off from theabsorption column 201 is 339 150 kg/h. It is taken off at a temperatureof 120° C. Only an amount of 336 690 kg/h having this temperature is fedto the spray cooler. 2 460 kg/h are fed to the redissociation.

The mixture of product gas mixture and quenching liquid which resultsfrom the direct cooling and has been cooled to 126° C. is fed as suchinto the bottom of the absorption column 201. The pressure in the bottomregion and in the spray cooler is 1.48 bar. The height of the absorptioncolumn 201 is 54.3 m.

The internal diameter of the absorption column 201 is 6.5 m in theregion of the Thormann trays and otherwise 6.0 m.

2460 kg/h of the bottom liquid taken off are fed to the dissociationcolumn 205 (consisting of a forced circulation flash evaporator and adual-flow tray rectification column seamlessly joined to the top ofthis). The number of dual-flow trays is 50. Like the absorption column201, the dissociation column 205 is insulated from the surroundings. Theinternal diameter of the dissociation column 205 is uniformly 2.4 m overall dual-flow trays. Its height is 27 m. The dual-flow trays arearranged equidistantly (400 mm) in the dissociation column 205. Theiropening ratio is uniformly 12%. Viewed from the bottom upward, the holediameter of the first eight dual-flow trays is uniformly 25 mm (holearrangement corresponding to strict triangular distribution) and thehole diameter of all subsequent dual-flow trays is uniformly 14 mm (holearrangement likewise corresponding to strict triangular distribution).The bottom liquid to be subjected to redissociation is fed in on theeighth dual-flow tray.

19 999 kg/h of a stripping gas stream which has been discharged at thetop of the absorption column 201 and subsequently superheated andcompressed (pressure: 2.9 bar; temperature: 157° C.) are fed into thebottom of the dissociation column 205. The composition of the strippinggas stream is:

0.269% by weight of acrylic acid,

0.090% by weight of acetic acid,

0.085% by weight of formaldehyde,

2.689% by weight of water,

0.009% by weight of formic acid,

0.08% by weight of acrolein,

0.001% by weight of propionoic acid,

0.001% by weight of furfurals,

0.001% by weight of allyl formate,

3.672% by weight of oxygen,

2. 517% by weight of CO₂,

0.779% by weight of CO,

0.095% by weight of propane,

0.212% by weight of propylene and

89.5% by weight of nitrogen.

513 646 kg/h of liquid phase having a temperature of 180° C. arecontinually taken off from the forced circulation flash evaporator. Ofthis, 512 997 kg/h are recirculated at a temperature of 180° C. to theforced circulation flash evaporator. The other 649 kg/h are degassed,diluted with methanol and passed to a residue incineration.

The dissociation gases formed in the forced circulation flash evaporatorare conveyed by means of the introduced stripping gas stream into thesuperposed rectification column and in this rise through the descendingrunback liquid.

An amount of 28 523 kg/h of a gas mixture (comprising stripping gasstream and dissociation gas) is discharged (temperature: 91° C.,pressure: 1.60 bar) from the top of the rectification column andrecirculated to the bottom region of the absorption column 201. Therecirculated gas mixture has the following composition:

28.8% by weight of acrylic acid,

0.219% by weight of acetic acid,

2.893% by weight of water,

0.018% by weight of formic acid,

0.081% by weight of formaldehyde,

0.057% by weight of acrolein,

0.041% by weight of propionoic acid,

0.028% by weight of furfurals,

0.001% by weight of allyl acrylate,

0.001% by weight of allyl formate,

0.004% by weight of benzaldehyde,

0.004% by weight of maleic anhydride,

2.54% by weight of oxygen,

1.765% by weight of CO₂,

0.546% by weight of CO,

0.066% by weight of propane,

0.149% by weight of propylene and

62.788% by weight of nitrogen.

As runback liquid, 6962 kg/h of mother acid from the crystallizationapparatus are recirculated to the uppermost tray of the rectificationcolumn comprising 50 trays.

The composition of the mother liquor is:

94.436% by weight of acrylic acid,

0.596% by weight of acetic acid,

3.788% by weight of water,

0.044% by weight of formic acid,

0.005% by weight of acrolein,

0.156% by weight of propionoic acid,

0.127% by weight of furfurals,

0.003% by weight of allyl acrylate,

0.001% by weight of allyl formate,

0.031% by weight of benzaldehyde,

0.040% by weight of maleic anhydride,

0.530% by weight of diacrylic acid,

0.139% by weight of polyacrylic acid (Michael adducts),

0.009% by weight of phenothiazine,

0.022% by weight of MEHQ,

0.072% by weight of other high-boiling constituents and

0.001% by weight of oxygen.

A centrifugal droplet separator is integrated into the bottom region ofthe absorption column 201 so as to prevent droplets of the bottom liquidbeing carried out in an upward direction from the bottom region.

The bottom region of the absorption column 201 is, as mentioned above,closed off by a first collection tray (chimney tray having 16approximately uniformly distributed roofed chimneys; chimney diameter:600 mm; chimney height: 1 m) at a column height (as in the case of allheights, calculated from the bottom tray upward) of 7.80 m.

The collection tray is configured with two walls and a 2° fall in aninward direction and provided with a central offtake cup and offtakeports (DN-200). The free gas cross section is about 30%. From this firstcollection tray, 83 559 kg/h of liquid are taken off as mentioned aboveand fed into the spray cooler.

The temperature at the bottom is 126° C. The pressure is 1.48 bar.

2.0 m above the first collection tray there is the first of initially 15dual-flow trays. These dual-flow trays (hole diameter uniformly 14 mm,number of holes uniformly 33 678, opening ratio uniformly 18%) arearranged equidistantly with a tray spacing of 380 mm. The throughopenings consist of circular openings having a uniform diameter of 14mm, with the stamping flash pointing downward in the separation column.The opening ratio is about 20%. The arrangement of the midpoints of thecircular openings follows a strict triangular distribution. The closestdistance between two midpoints of circles is 30 mm.

The fifteenth dual-flow tray is configured as a distributor tray. Forthis purpose, it comprises two plug-in tubes (DN-150) having 40 outflowholes (diameter: 15 mm) per plug-in tube.

The first series of dual-flow trays is ended by a second collection tray(chimney tray having 16 approximately uniformly distributed roofedchimneys; chimney height about 1.70 m, central offtake cup with offtakeports (DN-250), free gas cross section of 30%) which is located 1.50 mabove the last dual-flow tray. From this second collection tray, crudeacrylic acid having a temperature of 102° C. and the followingcomposition is continuously taken off at 1.47 bar:

97.076% by weight of acrylic acid,

0.4200% by weight of acetic acid,

1.614% by weight of water,

0.021% by weight of formic acid,

0.006% by weight of formaldehyde,

0.004% by weight of acrolein,

0.132% by weight of propionoic acid,

0.100% by weight of furfurals,

0.003% by weight of allyl acrylates,

0.0006% by weight of allyl formate,

0.025% by weight of benzaldehyde,

0.031% by weight of maleic anhydride,

0.543% by weight of diacrylic acid,

0.007% by weight of phenothiazine,

0.017% by weight of MEHQ and

0.0004% by weight of oxygen.

53 766 kg/h of the crude acrylic acid taken off from the secondcollection tray is recirculated directly to the absorption column 201 ata point below the dual-flow tray following the second collection tray inan upward direction.

91 152 kg/h of the crude acrylic acid taken off from the secondcollection tray are cooled to a temperature of 29° C. in a plurality ofstages by indirect heat exchange (preferably heat-integrated against themother acid to be recirculated to the absorption column 201). 1694 kg/hof the acidic water obtained at the top of the absorption column 201 arethen added to the cooled crude acrylic acid. The resulting mixture iscooled by further indirect heat exchange to 16.4° C. and then fed intotwo or three cooling plate crystallizers.

After partial crystallization, isolation and melting in hydraulicscrubbing columns, 20 973 kg/h of pure acrylic acid having the followingcomposition are taken off from the crystallization:

99.8247% by weight of acrylic acid,

0.1011% by weight of acetic acid,

0.0210% by weight of water,

0.0377% by weight of propionoic acid,

0.0001% by weight of furfurals,

0.0001% by weight of maleic anhydride,

0.0003% by weight of diacrylic acid and

0.0150% by weight of MEHQ.

It is outstandingly suitable for the production of superabsorbents basedon poly-Na-acrylate.

13 kg/h of PTZ are dissolved in 834 kg/h of the pure acrylic acid toproduce an inhibitor solution 1. 19 kg/h of MEHQ are dissolved in 30kg/h of inhibitor solution 1 to form the inhibitor solution 2.

The mother acid separated off in the hydraulic scrubbing columns isfirstly fed into a heatable collection vessel and from there into atank. From this, it is heated by heat integration to 90° C. andrecirculated in an amount of 65 038 kg/h to the fifteenth dual-flow trayof the absorption column 201 (counted from the top). The composition ofthis recirculated mother acid is as follows:

94.436% by weight of acrylic acid,

0.596% by weight of acetic acid,

3.788% by weight of water,

0.044% by weight of formic acid,

0.005% by weight of acrolein,

0.156% by weight of propionoic acid,

0.127% by weight of furfurals,

0.003% by weight of allyl acrylate,

0.001% by weight of allyl formate,

0.031% by weight of benzaldehyde,

0.040% by weight of maleic anhydride,

0.530% by weight of diacrylic acid,

0.139% by weight of polyacrylic acid (Michael adducts),

0.009% by weight of phenothiazine,

0.022% by weight of MEHQ,

0.072% by weight of other high-boiling constituents and

0.001% by weight of oxygen.

2.9 m above the second collection tray in the absorption column 201there is the first of 21 further dual-flow trays of the type describedabove (hole diameter again uniformly 14 mm, but number of holesuniformly 32 020 and opening ratio uniformly 17.4%) which were againarranged equidistantly with a tray spacing of 380 mm. The last of these21 dual-flow trays is configured as distributor tray having overflowgrooves with zig-zag overflow.

800 mm above the last dual-flow tray, the absorption column 201 beingsto widen conically. 500 mm above the last dual-flow tray, this wideningends at an internal column diameter of 6.50 m.

At this height, i.e. 1.50 m above the last dual-flow tray, anequidistant (tray spacing=500 mm) arrangement of 28 conventional,single-flow Thormann trays commences. The Thormann trays are configuredin such a way that an opposite flow direction of the liquid is achievedin each case in successive grooves in the crossflow direction by thearrangement of the driving slits in the caps of the Thormann trays.

The opening ratio of the Thormann trays is 14%. The ratio of chimneyarea to slit exit area is 0.8. The chimney height and the height of thedischarge weir is 40 mm. The tray freedom of the bubble cap (distancebetween lower edge of slit and tray) is 10 mm. The slit height is 15 mm.The angle between flared slit and longitudinal edge of the cap is 30° C.The length of the longitudinal edge of the cap is not more than 800 mm.In the peripheral region of the column, the cap length decreases down to200 mm in order to adapt to the roundness of the column. The distancebetween two caps located on one line in the cross-sectional direction is66 mm. The downflow area of the downcomer shaft is 1.5% based on thecross-sectional area of the tray. The width between the two lowerlongitudinal edges of a cap is 64 mm.

At the level of the uppermost Thormann tray, the separation column againbegins to narrow conically. 700 mm above the uppermost Thormann tray,this narrowing is at an end and the internal diameter of the columndecreases again to 6.00 m.

1.70 m above the uppermost Thormann tray there is the third collectiontray (chimney tray having 16 approximately uniformly distributed roofedchimneys, chimney height=1.50 m).

533 818 kg/h of acidic water having a temperature of 68.6° C. are takenoff at a pressure of 1.24 bar from the third collection tray. Thecomposition of the acidic water is:

12.23% by weight of acrylic acid,

4.00% by weight of acetic acid,

78.72% by weight of water,

0.70% by weight of formic acid,

0.09% by weight of formaldehyde,

0.01% by weight of acrolein,

0.01% by weight of propionoic acid,

0.0016% by weight of furfurals,

0.01% by weight of allyl formate and

4.23% by weight of methylene glycol.

29 821 kg/h of the acidic water (68.6° C.) taken off are recirculatedtogether with the inhibitor solution 2 to the uppermost Thormann tray.

60 kg/h of the inhibitor solution 2 are (viewed from the bottom)recirculated to the 19^(th) Thormann tray (at a temperature of 25° C.and a pressure of 1.10 bar).

6828 kg/h of the acidic water taken off are passed to incineration. 392000 kg/h of the acidic water taken off are recirculated at a temperatureof 36.6° C. to the sixth of the valve trays described below (calculatedfrom the bottom).

277 000 kg/h of the acidic water taken off are recirculated at atemperature of 31.4° C. to the uppermost of the valve trays describedbelow.

2300 mm above the third collection tray, there are eleven double-flowvalve trays arranged equidistantly (tray spacing=500 mm) in thecondensation column. The height of the discharge weir is 35 mm. Theopening ratio is 18% and the sum of the downflow areas of the downcomershafts of two successive valve trays is 10% of the column cross section.W12 valves from Stahl, Del., Viernheim, were used as valves. Thepressure at the top of the column is 1.2 bar.

169 164 kg/h of offgas having a temperature of 33.7° C. and thefollowing composition leave the top of the absorption column 201:

0.27% by weight of acrylic acid,

0.09% by weight of acetic acid,

2.79% by weight of water,

0.01% by weight of formic acid,

0.08% by weight of acrolein,

2.52% by weight of CO₂,

0.78% by weight of CO,

0.09% by weight of propane,

0.21% by weight of propylene

3.62% by weight of oxygen and

89.54% by weight of nitrogen.

The offgas is heated to 43° C. in an indirect heat exchanger and 94 691kg/h of this offgas are subsequently conveyed via a circulating gascompressor as diluent gas into the gas-phase oxidation and into theredissociation and 74 473 kg/h of the offgas are passed to incineration.

COMPARATIVE EXAMPLE

The comparative example is carried out essentially in the same way asthe example with the difference that no mother acid is introduced at thetop of the rectification column and the entire mother acid is conveyedonto the fifteenth tray of an absorption column and the runback to therectification column being formed by the gas mixture leaving therectification column being cooled to 61° C. at the top of therectification column by means of a superposed spray condenser. In thisway, 5663 kg/h were condensed out and supplied to the uppermost tray ofthe rectification column. A stabilizer solution comprising 0.98% byweight of phenothiazine dissolved in pure product were continuouslyintroduced into the spray condenser at a rate of 60 kg/h. The runbackliquid condensed out had the following composition:

95.10% by weight of acrylic acid,

0.05% by weight of furfurals,

0.02% by weight of acrolein,

0.03% by weight of formic acid,

0.52% by weight of acetic acid,

4.17% by weight of water,

0.1% by weight of propionoic acid and

0.01% by weight of phenothiazine.

All other process parameters were, if appropriate, kept constant. Themeasure resulted in the temperature at the bottom of the absorptioncolumn being reduced to 123° C. and the runback to the absorption columnbeing decreased to 27 899 kg/h. In the offtake stream from theabsorption column, the acetic acid content increased to 0.8% by weightand, as a result, a lower purity of acrylic acid of 99.75% by weightwith 0.18% by weight of acetic acid was obtained in the output from thecrystallization.

1. A process for recovering acrylic acid, which comprises the steps ofa) division of a heated mother acid stream from a crystallizationapparatus in the direction of an absorption column (201) and in thedirection of a dissociation column (205), b) feeding of the first heatedmother acid substream as runback to the uppermost tray of thedissociation column (205), c) feeding-in of at least one stripping gasstream below the lowermost tray of the dissociation column (205), d)feeding-in of a secondary component stream comprising oligomeric acrylicacid from the condensation column (201) to a middle tray of thedissociation column (205), e) dissociation of at least part of theoligomeric acrylic acid from the secondary component stream in thedissociation column (205) to give monomeric acrylic acid, f) removal ofthe secondary components comprised in the secondary component stream bycountercurrent rectification in the superposed dissociation column(205), g) discharge of the monomeric acrylic acid without condensationas gas mixture with the introduced circulating stripping gas stream atthe top of the dissociation column (205) and h) feeding-in of the gasmixture below the lowermost tray of the condensation column (201). 2.The process according to claim 1, wherein the mother acid stream isheated against an acrylic acid stream from a condensation column (201).3. The process according to claim 1, wherein step h) is carried outindirectly by the gas mixture being introduced into a quenchingapparatus (203) for quenching a product gas mixture comprising acrylicacid.
 4. The process according to claim 1, wherein the mother acidstream comprises essentially acrylic acid and proportions of water andacetic acid.
 5. The process according to claim 1, wherein the secondarycomponent stream comprises essentially acrylic acid, diacrylic acid andpolyacrylic acid and also proportions of maleic acid, benzoic acid,benzaldehyde, furfurals and water.
 6. The process according to claim 1,wherein the stripping gas stream comprises essentially nitrogen, acrylicacid, water and oxygen and also proportions of carbon dioxide and aceticacid.
 7. The process according to claim 1, wherein from 60% to 95%, inparticular from 85% to 90%, of the dissociable components of thesecondary component stream are dissociated in step e).
 8. A plant (1)for recovering acrylic acid, which comprises a condensation column(201), a dissociation column (205), a first line (101) connected to thedissociation column (205), a second line (102) connecting thecondensation column (201) and the dissociation column (205), a thirdline (103) feeding a substream of the mother acid obtained in thecrystallization into the dissociation column (205), a fourth line (104)connecting the crystallization apparatus and the absorption column (201)and a fifth line (105) connecting the dissociation column (205) and thecondensation column (201).
 9. The plant according to claim 8, whichfurther comprises a quenching apparatus (203) for quenching a productgas mixture comprising acrylic acid, which is arranged in the fifth line(105) between the dissociation column (205) and the condensation column(201).
 10. The plant according to claim 8, wherein the plant (1) isintegrated into an overall plant for the preparation of acrylic acid.